Because of its relatively low cost and inherently high strength to weight ratio, polyester molding compounds, including sheet molding compounds and thick molding compounds, are being utilized with increasing regularity in the automotive and appliance industry as a replacement for steel. These molding compounds generally comprise unsaturated polyester resin, a crosslinking agent such as styrene, reinforcing fibers such as glass fibers, fillers, a catalyst to enhance curing and various other additives. For applications wherein a smooth Class A surface finish is required, such as for automotive body panels, low profile additives which offset the shrinkage effects of thermosetting polyester molecules during crosslinking are used. These low profile additives counteract the effects of shrinkage to prevent the molding composition from pulling away from the mold surface which would cause waviness, voids or other surface irregularities.
Conventional low profile additives used for reducing shrinkage and for providing improved surface finish for thermosetting resin systems generally fall within one of two major categories. A first general category includes additives which are compatible with the precrosslinked thermosetting system. Examples of low-profile additives which are compatible with styrene crosslinkable unsaturated polyester systems prior to cure include polyvinyl acetate, polycaprolactone, and polyurethane. Compatible low-profile additives, particularly polyvinyl acetate, have been found to achieve excellent low-profile effect, exhibiting low or zero shrinkage and providing good surface appearance. However, such systems usually exhibit significant reductions in mechanical properties such as fracture toughness, tensile strength, modulus and strain to failure and the same is attributable to the morphology which arises from the mechanism by which low-profile effect is achieved. During the early stages of curing for a thermosetting resin system, employing a compatible low-profile additive, as crosslinking proceeds, a high molecular weight polymer network is formed which phase separates from the initial solution and forms spherical microgels or nodules with the initially compatible low-profile additive and unreacted crosslinker and resin forming a continuous phase. As the cure progresses to a more advanced stage, the nodules grow and form a macro-network structure wherein individual nodules are connected to neighboring nodules by comparatively small resin bridges, with the surrounding continuous phase, largely depleted of resin and crosslinker, now comprising mostly the low-profile additive. During the latter stages of the cure, the crosslinked polymer having a macro-network structure begins to shrink creating stresses which are transferred to the continuous, low-profile additive phase, causing the formation of micro-voids or cavitations in the low-profile additive continuous phase. The final cured composition can be described as having a popcorn-like structure or morphology wherein the crosslinked domains comprise nodules, each of which is coated with a thin film of the cavitated low-profile additive and is connected to neighboring nodules by relatively small bridges of crosslinked resin. The greatly reduced continuity of the phases and the ease with which the bridges can be broken explains the severe reduction in mechanical properties.
Thus, while conventional compatible low-profile additives such as polyvinyl acetate are very popular because of their excellent shrinkage compensation effects and because of the high quality surfaces produced therewith, their use is limited by the resulting morphology which leads to reduced toughness and strength.
The second major category of low-profile additives are those additives which are noncompatible with the initial precrosslinked thermosetting resin systems. Examples of noncompatible low-profile additives include polyethylene, polystyrene, polymethylmethacrylate, polyvinyl chloride, butadiene and block copolymers of styrene and butadiene. The noncompatible low-profile additives function differently from the compatible additives and provide an entirely different morphology and mechanism for achieving shrinkage compensation and low profile effect. Noncompatible low-profile additives when used with pre-crosslinked thermosetting resin systems become dispersed as discrete particles within a continuous phase comprising the crosslinker and crosslinkable resin, and ultimately provide a structure wherein the low-profile additive is dispersed as a discrete discontinuous second phase within a continuous crosslinked polymeric matrix. Noncompatible low-profile additives generally have been found to provide better mechanical properties than the compatible low-profile additives, but do not achieve -comparable low-profile effect. Other disadvantages with conventional noncompatible low-profile additives is that because they are noncompatible, i.e. insoluble, with the precured thermosetting solution, the low-profile additive particles can tend to segregate and agglomerate leading to nonuniform properties and surface appearance and paint adhesion problems. Moreover, such additives generally have high molecular weights and consequently form large particles, which, like agglomeration, create surface appearance and paint adhesion problems. Also, the large particles of conventional noncompatible low-profile additives may not be well bonded to the surrounding crosslinked polymer matrix leading to cavitation at the interface between the particles and the matrix, which is less effective for arresting crack propagation than cavitation within the particle.
Thus, while conventional noncompatible low-profile additives achieve a better morphology than conventional compatible low-profile additives, which leads to better mechanical properties, they do not provide very low or zero shrinkage as do the compatible additives. Also, typically they have surface appearance and paint adhesion problems which often preclude their use despite some improvement in mechanical properties.
Accordingly, given the present state of the art, it would be highly desirable to create an additive for thermosetting polymeric composition which counteracts shrinkage during the cure and provides a class A surface finish, while at the same time retaining the good mechanical properties of the polyester.